Electrochromic counter electrode

ABSTRACT

The present invention discloses an amorphous material comprising nickel oxide doped with tantalum that is an anodically coloring electrochromic material. The material of the present invention is prepared in the form of an electrode ( 200 ) having a thin film ( 202 ) of an electrochromic material of the present invention residing on a transparent conductive film ( 203 ). The material of the present invention is also incorporated into an electrochromic device ( 100 ) as a thin film ( 102 ) in conjunction with a cathodically coloring prior art electrochromic material layer ( 104 ) such that the devices contain both anodically coloring ( 102 ) and cathodically coloring ( 104 ) layers. The materials of the electrochromic layers in these devices exhibit broadband optical complimentary behavior, ionic species complimentary behavior, and coloration efficiency complimentary behavior in their operation.

CONTRACTUAL ORIGIN OF THE INVENTION

[0001] The United States Government retains fully paid licensing rightsin this invention pursuant to Contract No. DE-AC-36-99GO10337 betweenthe United States Department of Energy and The National Renewable EnergyLaboratory, a Division of Midwest Research Institute.

TECHNICAL FIELD

[0002] This invention relates to anodically coloring electrochromicmaterials, electrodes made therefrom, and their use in electrochromicdevices.

BACKGROUND ART

[0003] Electrochromic materials change optical properties upon theinjection of charge or charge carriers into the material, with thesubsequent movement of counterions or electrons to balance the charge inthe material. The mechanism underlying electrochromic behavior is notwell elucidated, and several theories are expounded in reviews of thesubject appearing in Advances in Image Pickup and Display, vol. 5, ed.B. Kazan, Academic Press, 1982, chapt. 5, and Display Devices, ed. J. I.Pankove, Springer-Verlag, NY, Berlin, Heidelberg, 1980. It is believedthat the effect primarily depends upon electron or hole charge injectioninto the electrochromic material, with the charge carrier beingdependent upon the nature of the material. Although counter ion movementis important to charge balance, it is not believed that coloring of thematerial is primarily dependent upon the nature of the counter ion. Inseveral investigations using a given electrochromic material, bothprotons and alkali metal cations yielded comparable color change andcharge dependent coloration. On this basis, substitution of the cationinjected within an electrochromic material seems not to have an effecton the quality of the color change in a given electrochromic material.It has been observed that different counter ions do effect the timerequired for a given electrochromic material to achieve a given level ofcoloration change.

[0004] Both inorganic and organic materials have been identified thatexhibit electrochromic behavior. Optical changes in electrochromicmaterials include a change from one color to another, a change fromtransmissive to reflective, and a change from transparent to opaque. Inthe electrochromic materials literature the terms often applied to thetwo states are “bleached” and “colored,” which refer respectively to acondition of low optical density and high optical density, even thoughsome materials have a “color” when in the low optical density state. Forthe purposes of this document, a material in the “bleached” state isalternatively described as being in a “bleached,” “low optical density,”“transparent,” or “high transmittance” state. In the same manner, forthe purposes of this document, a material in the “colored” state isdescribed alternatively as being “colored”, “opaque”, or in a “highoptical density” or “low transmittance” state. Again, for the purposesof this document, the process of changing from a “transparent” to an“opaque” state will be referred to as “coloring” and the process ofchanging from an “opaque” to a “transparent” state will be referred toas “bleaching”. Additionally, there are both cathodically and anodicallycoloring electrochromic materials (described in detail below).

[0005] For applications in which transparency is required, such asoptical shuttering or switching, both organic and inorganicelectrochromic materials have been employed, but for large devices suchas architectural windows, electrochromic materials made from inorganicmetal oxides are generally employed. Both anodically coloring andcathodically coloring electrochromic metal oxides have been identified.Anodically coloring electrochromic materials change from a low opticaldensity (or “bleached”) state to a high optical density (or “opaque”)state when electrons are withdrawn from the material (“holes” orpositive charge is “injected” into the material). In anodically coloringmaterials, coloring is accompanied by a migration of cations out of thematerial to balanced charge (holes) injected into the material.

[0006] Cathodically coloring electrochromic materials change from a lowoptical density (or “bleached”) state to a high optical density (or“opaque”) state when electrons are injected into the material. Incathodically coloring materials, coloring is accompanied by aconcomitant movement of cations into the material to balance chargeinjected into the material.

[0007] On a fundamental level, an electrochromic device may be thoughtof as a battery, wherein electrons and holes are moved within each “halfcell” associated with the anode and the cathode of an electrochromicdevice. To execute the transition from low and high optical densitystates, an electromotive force (EMF) such as an external battery isrequired to charge the electrochromic “battery” by forcing charge tomove within the device, driving the change in the electrochromicmaterials. The “charged” electrochromic device will, at least for theshort term, remain in that state until it is “discharged” by reversingthe EMF that drove the change or by short circuiting the device.Generally to speed the change and to insure that the device returns tothe lowest density achievable, it is preferable to drive the reversechange with a reversal of EMF rather than by simply shorting the device.

[0008] In the case of an anodically coloring material, a material iscycled from the bleached state to the colored state by connecting it“anodically” to a DC (direct current) source such that it supplieselectrons to the D.C. source (behaves as an anode). This is to say thatin an anodically coloring electrochromic material, the conductor inelectrical contact with the electrochromic material is connected to thecathode of a battery (or other emf source) to drive the material tochange to a colored state. The battery takes up electrons from theelectrochromic material. Of necessity, the counter-electrode of such adevice must be connected to the anode of the battery.

[0009] Bleaching in such a case is accomplished by reversing theelectrode connection of the device to the emf source. In such a case,the electrode in contact with the “anodically coloring” electrochromicmaterial would be connected to the battery anode. The electrochemicalmaterial would serve as an electron sink, and again of necessity thecounter electrode would be connected to the battery cathode, serving asa source of electrons. This scheme is reversed for a cathodicallycoloring electrochromic material.

[0010] Furthering the analog between an electrochromic device and abattery, the anode and cathode of the electrochromic material must beisolated from each other with respect to electron movement. If the anodeand cathode are well isolated, electrochromic coloring persists afterthe driving emf is removed from the device.

[0011] Unlike a battery, electrochromic devices preferably operate on aminimum of charge movement to effect a full change from bleached tocolored, whereas it is required that a battery store the maximumcharge/unit mass before it is saturated.

[0012] Electrochromic devices are generally layered structures. FIG. 1shows schematically from top to bottom the arrangement of the layers ofa typical electrochromic device 107. Generally, the top-most layer 106is a transparent support. Deposited on the lower face of the support, asit is depicted, is a first conductor 105, made from a transparentconductive material. Onto the exposed side of this first conductor isusually deposited a layer of electrochromic material 104. The exposedside of this electrochromic material is contacted with an electrolyte103.

[0013] The exposed side of the electrolyte layer is in contact with asecond electrode layer (102), which, depending upon the device may ormay not be electrochromic. A second electrically conducting layer 101 isin contact with electrode layer 102, which, depending upon the use towhich the electrochromic device may be put, may or may not betransparent. Many devices include a second protective layer, shown aslayer 100, which may or may not be transparent depending upon the use ofthe device.

[0014] The counter ion species necessary to balance charge movementwithin electrochromic materials is supplied by the electrolyte 103, anionic conductor in contact with the electrode (not shown), thecounter-electrode 102, or a source of ion dispersed within theelectrolyte (not shown). It is important to note that the interveninglayers between electrodes must at some point constitute a barrier toelectron flow between the electrodes (while providing an ion conductionpathway) or the device will short circuit and decolorize upon removal ofthe DC current source. In some electrochromic devices a layerspecifically designed to be an ion conductor and an electron barrier isincluded. Such devices generally employ liquid electrolytes between theelectrochromic layers and are not used in monolithic electrochromicdevices such as those of the present invention. Construction ofelectrochromic devices using a variety of electrochromic materials,electrolytes, electrodes, and device construction methods have beenreviewed in U.S. Pat. No. 6,020,987 to Baumann, et. al. and U.S. Pat.No. 6,111,684 to Forgette et al.

[0015] In devices which are designed to transmit light, the front andback electrodes are both made of a transparent conductor. An example ofsuch a transparent conductor is doped tin oxide glass, of which indiumdoped tin oxide glass is the most widely used. In such devices theelectrolyte and the source/sink of counter ion (if one is used) are alsotransparent. Electrolytes employed in electrochromic devices are wellknown in the electrochemical arts. Devices have been fabricated usingboth protic and aprotic medium in both liquids, polymers, and solidsolutions. Examples of liquid electrolytes include lithium perchloratein propylene glycol. Polymer electrolytes include, for example, theproton conductor family based on polyvinylpyrrolidone and other organichetroatom bases incorporated into polymers, and the alkali ionconducting polymers of which polyethylene oxide is the best known inrelation to electrochromic work. Examples of solid state ionicconductors that have been used as both ion sources and electrolytesinclude hydrated materials, for example SiO_(x) and MgF₂, and superionic conductors, for example sodium-b-alumina. Numerous other materialsin all of the above classes have been used to fabricate electrochromicdevices and are well known in the art. Each type has its own advantagesand disadvantages in terms of uniform color across a device, lowresistance to charge movement, ease of high volume fabrication, androbustness of the finished device. The prior art of this aspect ofelectrochemical device fabrication has been reviewed as well in U.S.Pat. No. 6,020,987 and references therein.

[0016] There are several consequences to the fact that coloring andbleaching are related to movement of charge within an electrochromicmaterial. The optical density of an electrochromic material in its“opaque” state is a function of the charge passed through the material.An electrochromic material which exhibits a high degree ofcoloration/unit of charge flow through the material is considered abetter electrochromic material than one that undergoes coloring to alesser degree with the same unit charge.

[0017] The rate of coloring and bleaching in an electrochromic materialseems to be a function of the rate of transport of cationic specieswithin the device as a whole. In general, devices using protons as acationic species having faster rates of coloring and bleaching thanthose using lithium, sodium, and other larger cationic species, allother factors being kept the same in a given device. When charge ispassed into or removed from an electrochromic material during coloring(depending upon whether it is a cathodically or anodically coloringmaterial), an emf arises across one or more of the interface regions ofthe device that opposes the direction of flow dictated by the appliedemf of charged species within the device. A direct consequence of thisis that the device must be operated at voltages exceeding the emfdeveloped in these space charge regions to insure complete chargesaturation of the device, and thus insure that maximum optical densityis achieved.

[0018] Often, voltages well above the space charge reverse emf must beemployed to attain acceptable rates of coloring to an acceptable levelof contrast between bleached and colored states of the device. Excessivevoltage, however, can lead to breakdown of the device as current passingthrough the device participates in electrochemical processes not relatedto coloring that result in the oxidation or reduction of constituents ofthe device and permanent alteration of the electrochromic material.

[0019] One aspect of electrochromic device fabrication employed toreduce the problem of high overvoltages is to fabricate the device witha non-polarizable counter electrode. Such an electrode is one thatfreely passes cationic species between the counter electrode and theelectrolyte of the device without building up a space charge region atthe interface. One example of a class of non-polarizable electrodes areelectrodes made from the tungsten bronzes, long used in the art asnon-electrochromic sources of counter ions.

[0020] Tungsten bronzes used as non-polarizable electrodes arecrystalline materials of primarily tungsten oxide containing a mobilecationic species. The tungsten bronzes are not electrochromic, and inthis respect are to be distinguished from cathodically coloring tungstenoxide electrochromic materials of the prior art. In general, thecationic counter ion species residing in the tungsten bronze isparticular to the given electrochromic material with which it is used ora counter electrode. Other non-polarizable electrodes which have beenemployed include oxides based on iridium and vanadium.

[0021] Transparent ionic conductors based on tantalum oxide have alsobeen employed as counter-electrodes. Examples of tantalum oxidecounter-electrodes, which act as a source and sink of counter ions butdo not participate in electrochromic coloration in the device, have beendisclosed in U.S. Pat. No. 4,832,468 to Ito et al., U.S. Pat. No.5,105,303 to Ilhage, and U.S. Pat. No. 5,831,760 to Hashimoto et al.Typical counter ions in these materials are protons and alkali metalssuch as sodium or lithium.

[0022] Non-polarizable electrodes permit an electrochromic device tooperate at coloration rates governed by the nature of ion flow in theelectrochromic layer used in the device, but do not enhance the degreeof coloration in a device in which they are employed. In this respectthey do not completely address the problem of slow coloration rates.

[0023] Some electrochromic materials, most notably organic materials,but also rhodium oxide-based materials, cycle between two differentcolored states. Some devices in which electrochromic materials may beemployed, such as optical switches and shutters, require that theelectrochromic material cycle between a “transparent” (low opticaldensity state) and an “opaque” (or high optical density state). Thesedevices must function across a broad optical spectrum. Materials whichexhibit two (or more) high optical density states in only narrowspectral bands are generally not suitable for such applications.

[0024] A material in a high optical density state may achieve its highoptical density by reflection, absorption, or a combination ofreflection and absorption processes. It has been observed in some typesof electrochromic materials that reflectivity is more related tocrystallinity and the morphology of the material than composition of thematerial. Thus, single crystal tungsten oxide is reflective in the“opaque” state and transparent in the “bleached” state, whilepolycrystalline films of this material in the “opaque” state seem tooperate by a mixture of reflection and absorption mechanisms. The“opaque” state of amorphous tungsten oxide films are thought to achievetheir opacity primarily by light absorption. This distinction isimportant in applications such as switching devices, where absorptiondissipates signal power, and electrochromic shutters on buildingwindows, where absorbed visible light can be re-radiated out the backside of the shutter as heat, negating the purpose of shuttering thewindow.

[0025] Tungsten oxide is a typical cathodically coloring material thathas been extensively studied. Iridium oxide is a typical anodicallycoloring material that has also been studied to a great extent. Theproperties of iridium and tungsten oxide electrochromic materials havebeen reviewed in Advances in Image Pickup and Display, vol. 5, ed. B.Kazan, Academic Press, 1982, pp 83-136.

[0026] Problems noted in this review relative to tungsten oxide devicesinclude solubility in some electrolyte systems (leading to poor storagelife) and cycling degradation, leading to short service life.Additionally, devices based on tungsten oxide that require both a highcontrast ratio (defined below) between “transparent” and “opaque” statesand a fast cycling time between the two states require operation atvoltages that accelerate physical degradation of the device, for exampleloss of adhesion between the electrochromic layer and the frontelectrode.

[0027] Additional observations regarding the nature of tungstenoxide-based electrochromic materials are disclosed in Display Devices,chapt 5. Faughnan and Crandall, Springer-Verlag Berlin. The authors ofthis work teach that it is necessary to incorporate water into tungstenoxide-based films before they exhibit electrochromism. Films formed fromgelation of aqueous tungsten oxide precursors display superiorelectrochromic properties than those formed by reactive sputteringtechniques. These authors also observe that amorphous tungsten oxidefilms exhibit faster coloration than crystalline films. They suggestthat this is because of more facile ion migration in the amorphousmaterial than is possible in crystalline materials. They also note thatthe amorphous films display some loss of maximum optical density anddensity bandwidth relative to results obtained with polycrystallinefilms and single crystal materials.

[0028] Electrochromic devices have been predicated on these principals,thus in U.S. Pat. No. 4,889,414 to Rauh et al., an electrochromic deviceis disclosed in which a highly ordered polycrystalline tungsten oxidefilm is employed to act as a wavelength variable reflectance device. Thewavelength of light reflected by the device is adjusted by regulatingthe amount of charge injected into the electrochromic material.

[0029] In their review of work done with iridium oxide-basedelectrochromic devices, the reviewers noted that the iridium oxide-basedelectrochromic material appeared more robust to mechanical and cyclingdegradation than tungsten oxide-based materials. Additionally, theiridium oxide-based material had a more neutral color, and executedcolor/bleach cycles at a faster rate than tungsten oxide-basedmaterials. The reviewers also disclosed that the ultimate opticaldensity, and the density developed by the material as a function of theunit of charge passed through the material (the so called colorationefficiency) is lower for the iridium oxide-based material than for thetungsten oxide-based material.

[0030] As noted above, iridium oxide-based materials are anodicallycoloring electrochromic materials. As with cathodically coloringmaterials, the mechanism by which the anodically coloring materialscolor is in dispute. Suggested mechanisms range over the samepossibilities for anodically coloring materials as for cathodicallycoloring materials. While movement of ions into or out of iridiumoxide-based materials seems to be important to the mechanism by whichthese materials undergo coloring; changing the species employed as acounter anion in the anodically coloring materials does not have asdramatic an effect on the coloring/bleaching cycle rate as it does inthe case of the cathodically coloring materials.

[0031] As for the tungsten oxide-based materials, the iridiumoxide-based materials require hydration before they are an effectiveelectrochromic material. It has been suggested that the transport ofanions into the material is an important factor in the coloringmechanism. Iridium oxide-based-materials seem to be insensitive to thenature of the anion employed to balance charge, thus sulfate, fluoride,and hydroxide media all appear to be equally effective counter ions.This has caused other investigators to suggest that the generalinsensitivity to electrolyte character makes it difficult to distinguishbetween an electrochromic process in which an anionic species istransported into the material to balance charge, and one in which acationic species is transported from the material to balance chargeduring coloring.

[0032] The characteristics of merit in an electrochromic material arerelated to its ability to color and its durability. The characteristicsthat are usually of interest relative to a material's ability to colorare: the maximum optical density achievable by the material; thecontrast between minimum and maximum optical density; the rate at whichthe material can be cycled between bleached and colored states; and thecurrent required to achieve a specific optical density in the coloredstate.

[0033] Maximum and minimum optical density, and contrast between thebleached and colored states can be determined using conventional opticaldensitometry methods, or by optical spectroscopy wherein the filmdensity is expressed in either percent transmission units or inabsorbance units.

[0034] The current required to reach a given level of coloring can beexpressed as the Coloration Efficiency (CE) of the material. ColorationEfficiency is expressed as a function of the change in transmittance ofa sample per unit charge passed through a unit area of an electrochromiclayer according to the equation:

CE=Log(τ_(b)×τ_(c) ⁻¹)×ΔQ ⁻¹

[0035] Where CE is coloration efficiency, τ_(b) and τ_(c) are theoptical transmittance of the device in the bleached and colored statesrespectively, and ΔQ is the unit area coulombic charge passed throughthe device. As is conventional, optical transmittance is expressed as apercentage.

[0036] Percent transmittance is calculated in the common manner, bydividing the flux of the light passing through the device by that of thelight striking the front side of the device. The expression ΔQ iscalculated by measuring the amount of charge passed through the deviceto change it from a bleached to the colored state, and then dividing bythe area of the electrochromic layer that has received that charge.Generally, coloration efficiency (CE) is expressed as the integratedchange in optical density over a specified range of wavelengths (orchange in optical density for a given spectral region) One spectralregion of interest in electrochromic devices is the visible region whichis defined as the range of wavelengths from 400 to 700 nanometers.

[0037] Durability of electrochromic films is generally determined byincorporating a specific electrochromic material into a device such as ashutter and subjecting it to numerous coloring/bleaching cycles under avariety of emf conditions. The number of cycles a device can undergobefore it is degraded (usually determined optically) is one importantparameter that can be determined in this manner. Another importantcharacteristic that may be ascertained in this way is the ability of amaterial to sustain a high number of coloration/bleach cycles withoutlosing contrast between the states. Still another characteristic is theability of the device to maintain an optically dense state without thecontinued application of an emf. The ability of the device to be storedquiescent without degradation of the device (shelf life) is also animportant characteristic which may be determined in this manner.

[0038] In general, a practical electrochromic device should be able toachive a large optical density increase (called contrast) between itsbleached state and its colored state. It is desirable for devices toexhibit their maximum optical density at a low injected charge value,usually calculated in terms of unit area of electrochromic material.Practical electrochromic devices should operate over tens of thousandsof cycles without degrading below usability, and should be able towithstand long periods of storage without degradation (the devicesshould be stable). Finally, as an extrinsic property, a device utilizingan electrochromic material should be able to achieve maximum opticaldensity in time scales on the order of seconds to minutes depending uponthe application and device size.

[0039] Researchers have found that, in terms of practical devices,maximum optical density is an intrinsic property of the electrochromicmaterial. While greater density can be achieved by utilizing thickerfilms, thicker films must either be operated at higher over-voltages(voltages in excess of the minimum required to cause charge flow in thedevice) or require very long bleaching/coloration cycles. Highover-voltages lead to irreversible secondary reactions occurring withinthe device. These secondary processes can lead to reduced contrastbetween high and low optical density states and a less durable device.Long cycle times for the coloration/bleach cycle yields devices whichare not practical. In this respect, practical devices are thereforelimited to a maximum film thickness and a material dependent maximumoptical density in the opaque state.

[0040] In general, electrochromic materials which exhibit a high ratioof optical density increase per injected charge (high colorationefficiency) can be operated at low over-voltage potentials. This featureyields devices with longer cycle lives. Such materials generally afforddevices which can achieve a high level of optical density and which haveshort coloring/bleaching cycle times. For these reasons, such materialsare preferred in the construction of practical electrochromic devices.

[0041] Several practitioners skilled in the electrochromic arts havesuggested that this problem could be addressed by constructing a deviceutilizing two electrochromic layers (one anodically and one cathodicallycoloring layer) stacked along the optical path of a single device, eachlayer augmenting the absorption characteristics of the other. An exampleof such a device is disclosed by Lampert in a review of electrochromicmaterials suitable for windows (Lampert, C. M., Solar Energy Materials11 (1984) 1-27). In theory, this type of construction yields a devicethat increases the optical density achievable with a given charge passedthrough the device over what is possible in a device using a singleelectrochromic layer. This is particularly useful when fabricatingshutter type electrochromic devices wherein colored state opticaldensities are desired that approach 20% or less transmittance.Additionally, a device with an electrochromic layer on both electrodesdoes not have a significantly greater optical density in the bleachedstate than a device using only one electrochromic layer and atransparent counter electrode.

[0042] Electrochromic materials may demonstrate absorption curves thatare flat within a narrow spectral region, however, over a broad spectrumin general they exhibit absorption curves that are not flat. It isunlikely that two different materials can be found that perfectlyaugment each other across a broad spectral band. Thus, at best, twodifferent electrochromic materials would only be additive within a givennarrow spectral band. However, using this same concept, two differentelectrochemical materials may be superimposed to compliment each other,thus expanding the spectral range over which such a device might haveuseful function as a shutter. Such materials exhibit complimentaryelectrochromic behavior. In such a situation, each could also contributeto some small degree to the overall optical density of the device acrossa broader spectral region (each layer augmenting the absorption of theother) but the primary effect would be to expand the spectrum over whichthe device could function rather than to augment the density the devicecould achieve in a narrow region of the absorption spectrum.

[0043] In a device having two superimposed complimentary electrochromiclayers, the coloring cycle also utilizes the ions migrating out of theanodically coloring layer to supply the ions required for charge balancein the cathodically coloring layer, thereby utilizing the movement ofone charged species to increase the optical density of eachelectrochromic layer in the device, albeit to different effect across aspectral region. This method both increases the maximum optical densityachievable with the device and reduces the time required to execute acoloring/bleaching cycle to or from a given optical density. The maximumoptical density achievable by such a device is increased because withina given spectral band the optical density increase in each of the twoelectrochromic layers is added together.

[0044] The time required to execute a bleaching/coloring cycle isrelated to the diffusion rates of charged species in the electrochromiclayers of a device. A device utilizing two complimentary electrochromiclayers reduces this time for two reasons. The first process by which adevice with complimentary electrochromic layers reduces cycle time alsorelates to the optical density of the two electrochromic layers beingadditive. In such a case, the amount of charge which must be moved toachieve a given density across the whole device is less than would berequired in a device having a single electrochromic layer. Since lesscharge must be moved, the coloring/bleaching time is shorter.

[0045] The second reason that a device having complimentaryelectrochromic layers reduces the cycle time is related to the effect ofmoving charge within the device. As charge is moved during a coloringcycle, the space charge region increases across the device, andresistance to current passing through the device increases. Thus, as thedevice is colored to an increasing level, it takes increasingly longerto pass a unit charge through the device.

[0046] A plot showing unit charge passed/unit time as a function oftotal amount of charge moved within a material would give a curve ofdecreasing value of charge moved/unit time as increasing amounts ofcharge were moved for a given applied emf (basically, as the charge isbuilt up in the space charge region there is greater resistance toadditional charge flow). A device having two electrochromic layersoperates in a lower net moved charge end of this curve (wherein lesstotal charge has been moved, thus the resistance to movement of chargeis relatively low and charge transport is more facile). This means thatcoloration/bleaching cycles requiring a total movement of charge withintwo different layers occurs over a shorter period of time than movingthe same amount of charge in a device having a single electrochromiclayer for a transition from a bleached state to any given opticaldensity. While this notion has been shown to be promising, problemsrelated to slow cycle rates still exist with the prior art materials.

[0047] Materials disclosed or suggested in the prior art to address theproblems posed by electrochromic materials which either compliment oraugment each other (or both compliment and augment each other to varyingdegrees) only achieve either end over a narrow spectral range. The basisof the problems displayed by prior art materials rest with therequirements which must be met for electrochromic materials to becomplimentary. For an anodically coloring material to be complimentaryto a cathodically coloring material, the two materials must have severalcomplimentary properties. The two materials must be complimentary interms of optical properties, in terms of ionic species transported bythe electrochromic material in undergoing a bleaching/coloring cycle,and in terms of having approximately the same coloration efficiencies.When these characteristics are matched in a given device, the devicewill achieve maximum optical performance from the electrochromicmaterials utilized. These multiple characteristics are described hereinby the phrases “broadband optically complimentary,” “ionic speciescomplimentary,’ and “coloration efficiency complimentary” behavior.

[0048] The concept of an “broadband optically complimentary” materialhas not been heretofore addressed by any of the prior art. Broadbandoptically complimentary electrochromic materials exhibit absorptionspectra such that for a given spectral band the sum of the absorbancesof the two materials across that spectral band is approximatelyconstant. In such a situation, the areas of decreased absorptionefficiency in one material coincide with areas of increased absorptionefficiency in the other material.

[0049] This is best illustrated with reference to FIG. 9. FIG. 9 showsoptical transmittance as a function of wavelength for a tungsten oxide,a cathodically coloring electrochromic material, and nickel tantalumoxide, an anodically coloring electrochromic material. The transmissionspectra of these materials after receiving similar amounts ofelectrochemically injected charge are overlayed. It can be seen thatwhere the cathodically coloring material has high transmittance, theanodically coloring material exhibits low transmittance, and vice-versa,illustrating broadband optically complimentary materials.

[0050] Two materials can be said to be “ionic species complimentary” ifduring a coloration/bleach cycle ionic current flow for both materialsis in the same direction and the electronic current flow for bothmaterials is in the same direction. By way of illustration, anelectrochromic device employing a cathodically coloring material and ananodically coloring material can be expressed as the following sum ofhalf reactions:

Li_(y+z)WO_(x)(colored)

Li_(z)WO_(x)(bleached)+ye⁻+yLi⁺//Li_(d)W_(a)Ni_(b)O_(c)(colored)+ye⁻+yLi⁺

Li_(d+y)W_(a)Ni_(b)O_(c)(bleached)

[0051] Where WO_(x) is a tungsten oxide-based electrochromic materialand W_(a)Ni_(b)O_(c) is a tungsten-doped nickel oxide-basedelectrochromic material. In this illustration, during a coloring cycle,the electrode in contact with the tungsten-doped nickel oxideelectrochromic material acts as an anode, supplying electrons to thebattery driving the electrochromic change via the conductor in contactwith the electrochromic material. The doped nickel oxide-basedelectrochromic material itself acts as an ion source, supplying lithiumto the tungsten oxide-based electrochromic material. Both thetungsten-doped nickel oxide and the tungsten oxide-based electrochromicmaterials undergo coloring in the process. The electrode attached to thetungsten oxide-based electrochromic material acts as a cathode,accepting electrons from the battery driving the electrochromic changeand in turn supplying them to the tungsten oxide-based electrochromicmaterial. The tungsten oxide-based electrochromic material itself actsas an ion sink, accepting lithium ions from the doped nickel oxide-basedelectrochromic material. Although this process is generally driven by abattery, any emf source wherein the positive pole of the emf wasattached to the electrochromic device anode and the negative pole of theemf device was attached to the electrochemical device cathode drivessuch a device from a bleached to a colored state. Reversing thisconnection drives the material from a colored to a bleached state.

[0052] In this example, the materials are “ionic species complimentary”in that one ejects lithium at the same time the other injects it duringthe coloring/bleaching cycle. It would be possible to fabricate a devicein which an intermediary ion conductor was used to essentially isolatethe half reactions, making it possible for different ionic species to beused in each electrochromic layer to balance charges, but in such asituation the electrochromic materials could not be said to exhibit“ionic species complimentary” behavior. It is also a requirement thatmaterials can only exhibit “ionic species complimentary” behavior if oneof them exhibits anodically coloring behavior and the other exhibitscathodically coloring behavior. No “ionic species complimentary”behavior is possible if two materials both color anodically or bothcolor cathodically.

[0053] With reference to FIG. 3, the absorption curves of prior artanodically coloring electrochromic materials based on nickel oxide andtungsten nickel oxide are compared with a cathodically coloringelectrochromic material based on tungsten oxide according to theircoloration efficiency. Coloration efficiency has been defined above.

[0054] Tungsten oxide is an electrochromic material frequently employedin electrochromic devices. It can be seen from an examination of FIG. 3that although relatively flat, the coloration efficiency of the nickeloxide is poor across the wavelengths spanning the violet through the redportion of the visible light spectrum, and as stated above is poorlymatched to Tungsten oxide, which has a coloration efficiency about 2-10times that of the nickel oxide in the spectral region between 400-700 nm(visible spectrum). Tungsten nickel oxide has a better colorationefficiency in the blue region of the spectrum than either tungsten oxideor nickel oxide, but falls off in the yellow to red region as tungstenoxide increases in coloration efficiency. All materials are poorabsorbers in the ultraviolet and blue end of the visible spectrum. Inthe particular example given, a device using a tungsten-doped nickeloxide-based electrochromic material in conjunction with a tungstenoxide-based electrochromic material would exhibit “ionic speciescomplimentary behavior” but not “optical complimentary behavior” or“coloration efficiency complimentary behavior”. Devices utilizing thismaterial in a device employed to shutter sunlight, for example, wouldoperate poorly in this region as they would change the color of thetransmitted light as a function of the depth of coloration of theelectrochromic device.

[0055] The concept of anodically and cathodically coloringelectrochromic materials employed in the same optical pathway of anelectrochromic device has been disclosed in the prior art. Thus U.S.Pat. No. 5,777,780 to Terada et al. disclose a sealing system which canbe used with any type of electrochromic device, including ones thatemploy anodically and cathodically coloring electrochromic materials inthe same optical path. This patent discloses compositions used forsealing such devices. The patent provides examples of electrochromicdevices prepared by reactive sputtering of iridium onto a transparentelectrode to form a film of hydrated iridium oxide. A tantalum oxidelayer is then applied to the iridium oxide layer by vacuum depositionfrom a tantalum oxide source. This layer acts as a counter ionsink/source for electrochromic changes in the device. A secondelectrochromic layer based on tungsten was then deposited followed byanother layer of transparent conductive material. This device was nottested for complimentary electrochromic behavior as that term is definedand disclosed elsewhere in this document, only for raw ability torepeatedly undergo coloring/bleaching cycles. Additionally, this patentdiscloses a device using a single electrochromic layer based on tungstenoxide and a vacuum deposited layer of tantalum pentoxide as an ionsource and counter electrode. This patent also discloses that thesubject sealing system will work equally effectively with devices ofother construction, including devices that employ intermediate layerscontaining tantalum pentoxide in mixtures with group 8 metal oxides andhydroxides or in mixtures with materials of known electrochromicbehavior. It does not disclose how to make such materials, but teachesthat in these constructs the tantalum pentoxide maintains a discretecrystalline nature in the mixtures and itself is not electrochromicallyactive.

[0056] U.S. Pat. No. 4,889,414 to Rauh et al. disclose electrochromicdevices which employ both anodically and cathodically coloringelectrochromic materials. This patent discloses devices made of avariety of combinations of anodically and cathodically coloringelectrochromic materials but does not address the question of broadbandoptically complimentary behavior and coloration efficiency complimentarybehavior in the combinations of materials used, reciting materialcombinations on the basis of merely ionic complimentary behavior, whichhas been shown by earlier workers to be an inadequate basis upon whichto select combinations of electrochromic materials for use in practicaldevices. The only tantalum containing electrochromic compounds disclosedare sulfide and selenide layered materials. All of the devices aredirected to single crystal or highly oriented polycrystalline films.

[0057] Reissued U.S. Patent Re 34,469 to Cogan et al. also disclose anelectrochromic device based upon a layer of anodically coloringelectrochromic material and a layer of cathodically coloringelectrochromic material superimposed in the optical path of the device.In the disclosed examples, tungsten oxide-based electrochromic materialis the preferred cathodically coloring material disclosed. Anodicallycoloring electrochromic materials used in example devices includematerials based on vanadate and chromate. This patent teaches thatmixtures of tantalum oxide with a vanadate, or chromate, or mixedvanadate/chromate electrochromic anodically coloring material will alsofunction in the disclosed device. Methods of making the metal oxidemixtures are not disclosed. Additionally they disclose that tantalumoxide may be used as a cation source/sink in the disclosed devices.

[0058] This patent addresses ionic species complimentary behavior andcoloration efficiency complimentary behavior without addressingbroadband optical complimentary behavior. In the examples presented, thevanadate and chromate based anodically coloring materials displaycoloration efficiency that is 1-10% that of the cathodically coloringtungsten oxide-based material. No evidence of broadband opticalcomplimentary behavior is presented in the disclosed devices. Thispatent discloses that adequate films of the claimed composition may beformed utilizing any of the known film forming techniques, includingsputtering, vapor deposition techniques of all types, andcoating/precipitation techniques of all types.

[0059] Additionally, this patent discloses that any manner of injectingcounter ions into an electrochromic precursor material will produce anequally satisfactory electrochromic material. Thus for example, exposinga metal oxide film to electrochemical redox processes employing lithiumions produces a material equally satisfactory as an electrochromicmaterial to the same metal oxide film exposed to lithium vapor in avapor infiltration process.

[0060] Other examples of mixed oxide-based electrochromic material havebeen reported. Thus U.S. Pat. No. 4,282,272 to Matsuhiro et al. disclosea device employing a single layer of tungsten oxide-based electrochromicmaterial into which tantalum oxide has been. These investigatorsincorporate tantalum oxide into tungsten oxide films by vacuumevaporating a mixture of tungsten and tantalum oxide onto a substrate.No improvement in electrochromic behavior was noted, but improved heatstability of the resulting mixed oxide films.

[0061] U.S. Pat. No. 4,365,870 to Morita et al. discloses anelectrochromic cermet of mixed tungsten oxide and tantalum oxide. Thismaterial is prepared by sputtering from a tungsten oxide target thatsupports a piece of tantalum metal as well. These workers noted that thespace charge emf builds up less rapidly for a given charge passedthrough a device made from the mixed oxide cermet than for an equivalentdevice made from the tungsten oxide alone. This they equate withimproved colored state stability and more rapid response.

[0062] Various techniques have been employed to form films ofelectrochromic material. One technique used frequently in earlyinvestigations is a mechanical application of a parent species to asubstrate, such as spin or dip coating or spray drying material. Anexample of this is the preparation of electrochromic tungstenoxide-based films by spray-drying metatungstic acid (H₆W₁₂O₃₉) onto hotquartz. Using this method, films 2-5 micron thick and havingsystematically variable water content can be prepared.

[0063] Another technique used to prepare electrochromic films iselectrochemically precipitating a coating of a precursor material ontoan electrode, or electrochemically modifying an existing coating on anelectrode. Thus, iridium oxide films have been prepared on transparentconductors starting with a coating of iridium metal applied to theconductor using an evaporative coating process. The electrode (iridiumon transparent conductor) was then subjected to oxidation in anelectrochemical process using sulfuric acid as an electrolyte to form aniridium oxide film. The film was subjected to further electrochemicalprocessing to injection cations into the iridium oxide film, therebyrendering it electrochromic. This has been carried out using transparentelectrodes such as indium tin oxide coated glass.

[0064] Electrochromic layers have also been prepared utilizingconventional thin film vacuum preparation techniques such as sputtering,coating by evaporation of a source material, and chemical vapordeposition. Electrochromic films deposited in this manner include V₂O₅,MoO₃, Nb₂O₃, and IrO₂.

[0065] Hydrated nickel oxide has been known for some time as ananodically coloring electrochromic material. Its electrochromicproperties have been studied and reviewed (Lampert, “ElectrochromicMaterials and Devices for Windows,” Solar Energy Materials, 11 (1984) pg16). It has been disclosed that nickel oxide films with lithium injectedelectrochemically into them can be used to fabricate electrochromicdevices. The native nickel oxide film is first bleached byelectrochemical reduction of the material. Lithium ions migrate into thestructure during reduction where they apparently participate in arestructuring of the film.

[0066] At some point during ionic species injection, maximumtransparency of the film is achieved, and further reduction results incoloration of the film as additional lithium ions are inserted. Thelithium ions inserted during the initial bleaching stage (reduction ofthe material) do not readily migrate from the material when it issubsequently subjected to an oxidation. Lithium ions inserted into thematerial after the point of maximum transparency are readily removedupon subsequent electrochemical oxidation of the lithiated nickel oxidefilm. This oxidation step is accompanied by a subsequent bleaching ofthe material.

[0067] Once prepared, the material can be cycled reversibly betweencolored and bleached states with the accompanying movement of lithiumions in and out of the electrochromic material. The migration of lithiumions out of the structure under reduction does not proceed past thepoint of maximum transparency unless extraordinary voltages are appliedto the material (generally in excess of 4 Volts relative to Li). Thus,lithiated nickel oxide prepared in this manner defines an electrochromicmaterial which exhibits anodic coloring.

[0068] The authors of this work, F. Decker et. al., Electrochima Acta,vol 37, 6, (1992), pp 1033-38, did not determine the stoichiometry oflithium ions to nickel metal centers in the nickel oxide films at thepoint the films changed from bleaching to coloring with further lithiumion insertion. They also noted that the rate of coloring and bleachingwas tied to the rate of ion migration, which was thought to berelatively slow.

[0069] The authors of this work have further disclosed that an electrodeprepared with a lithium ion injected nickel oxide thin film can beincorporated into an electrochromic device. The authors prepared adevice using a lithiated nickel oxide counter electrode and a tungstenoxide electrode as the primary electrochromic material. As disclosedabove, tungsten oxide is a cathodically coloring electrochromicmaterial, in this case becoming optically denser upon reduction, and thesubsequent migration of lithium into the material.

[0070] In this device, the lithium migrating into the tungsten oxide isthe lithium which was ejected by the lithiated nickel oxide electrodeduring its oxidation. The net result is that both materials undergocoloring with the movement of a single electron and a single lithiumion. This example device can not be operated as a practical device. Thelithiated nickel oxide electrode requires current density far in excessof the tungsten oxide to undergo coloring or bleaching. In practice, adevice made using a lithiated nickel oxide counter electrode wouldrelegate the nickel oxide-based material to a role of primarily actingas a sink for lithium ions rather than as a significant contributor ofoptical density to the device. The authors also noted that slow kineticsalso hamper the practical utility of this device. Apparently, because ofthe nature of the rate of charge transfer in the lithiated nickel oxidefilms, lithium transport in the device is slow, thus the transition timebetween the bleached and colored states is too long to be useful forpractical applications.

[0071] Other workers have disclosed attempts to address the problem ofslow coloring/bleaching cycle times in nickel oxide materials. Lee et.al. has disclosed a nickel oxide film which incorporates tungsten (Lee,Sh and Joo, S K, Solar Energy Materials and Solar Cells, 39 (1995), pp155-66). These workers reasoned that incorporation of W⁺⁶ atoms (whichhave an atomic radius similar to that of Ni⁺²) would be facile and thatdoing so would result in two nickel vacancies for every tungstenincorporated into the nickel oxide lattice on the basis of chargeneutrality.

[0072] On this basis, Lee and Joo prepared tungsten nickel oxide byreactive ion sputtering from a nickel target containing tungstensegments in an oxygen atmosphere (Lee, S H and Joo, S K, Solar EnergyMaterials and Solar Cells, 39 (1995) pp. 155-166). The tungsten nickeloxide thus produced was deposited onto a transparent conductor forming atungsten-doped nickel oxide electrode.

[0073] The term doped material is applied herein when it is meant thatthe moiety referred to as a dopant is substitutionally incorporated intothe structure of the material in which it is considered a dopant, and ispart of the amorphous structure of the material into which it has beenincorporated. As used herein, dopant materials are present inconcentrations ranging from a fractional atomic percent to 50 atomicpercent of the composition of the doped material.

[0074] When a tungsten-doped nickel oxide thin film electrode preparedaccording to the process of Lee and Joo was electrochemically injectedwith lithium ions, the resulting material was shown to be capable ofanodically coloring. A device was prepared utilizing this material. Thematerial colors upon oxidation with the subsequent ejection of lithiumions out of the electrochromic material. A second thin filmelectrochromic device was fabricated substituting the tungsten-dopednickel oxide-based counter electrode for a counter electrode based on athin film of nickel oxide alone (no tungsten-doped into the nickeloxide). This electrode was also lithiated using the same procedure bywhich the tungsten-doped material was lithiated.

[0075] These two devices were compared electrochemically. It wasobserved that after the first lithium insertion cycle, the tungstennickel oxide displayed a high lithium ion current at loweroverpotentials than was required for the nickel oxide alone.Additionally, with reference to FIG. 3, the tungsten-doped nickel oxideelectrode displayed a higher coloration efficiency (CE, defined above)than the undoped nickel oxide electrode.

[0076] Further study of the tungsten-doped nickel oxide films (Lee, S H;Park, Y S; and Joo, S K, Solid State Ionics, 109 (1998) pp 303-310)showed that the addition of tungsten to nickel oxide produced anelectochromic material that executed coloring/bleaching cycles morerapidly than what is observed for undoped lithiated nickel oxide films.These investigators also noted that increasing the percentage oftungsten incorporated into nickel oxide films resulted in increasingelectrical resistivity of the material, but the resistance of thematerial to ionic species migration, specifically the migration oflithium cations, was more facile than that observed for the film ofundoped nickel oxide.

[0077] In further investigatory work, a tungsten-doped nickel oxideanode was used to fabricate an electrochromic device and compared with asimilar device utilizing an undoped nickel oxide film. The device withthe doped nickel oxide showed improved contrast between low and highoptical density states, and displayed a more rapid coloring cycle thanthe undoped material.

[0078] Electrochromic devices fabricated with nickel oxide films asanodes do not execute a coloring/bleaching cycle sufficiently rapidly orat sufficiently low enough over-potentials to be commercially viable.Further, the performance characteristics of undoped nickel oxide filmspreclude their use as an electrochromic material that can contribute ina meaningful way to the overall contrast of the device when used inconjunction with a cathodically coloring electrochromic film.

[0079] Although the tungsten-doped nickel oxide films show improvementwhen compared with the undoped nickel oxide films, the devices assembledwith them to date exhibit less than desirable performancecharacteristics when used in conjunction with cathodically coloringmaterials such as tungsten oxide. In such devices, optimum colorationefficiency complimentary behavior, and optimum broadband opticalcomplimentary behavior are lacking, adversely effecting the performanceof the device.

[0080] The present invention addresses the problems of slow iontransport kinetics and low coloration efficiency not addressed by theseother materials. The present invention additionally provides a materialthat can exhibit broadband optical complimentary behavior, ionic speciescomplimentary behavior, and coloration efficiency complimentary behaviorwhen used in conjunction with anodically coloring prior artelectrochromic materials.

DISCLOSURE OF INVENTION

[0081] One aspect of the present invention is the production of anelectrochromic material that will cycle from a condition of low opticaldensity to a condition of high optical density when employed as an anodein an electrochemical device.

[0082] Another aspect of the present invention is to produce a thin filmof electrochromic material with a controlled ratio of constituents.

[0083] Another aspect of the present invention is to produce anelectrochromic material that is the optical and electrical compliment ofa tungsten oxide electrochromic material.

[0084] Other aspects of this invention will appear from the followingdescription and appended claims, reference being made to theaccompanying drawings forming a part of this specification wherein likereference characters designate corresponding parts in the several views.

BRIEF DESCRIPTION OF DRAWINGS

[0085] Before explaining the disclosed embodiment of the presentinvention in detail, it is to be understood that the invention is notlimited in its application to the details of the particular arrangementshown, since the invention is capable of other embodiments. Also, theterminology used herein is for the purpose of description and not oflimitation.

[0086]FIG. 1A Cross-sectional Diagram of the Layers Comprising anElectrochromic Device.

[0087]FIG. 2A Cross-sectional Diagram of an Electrochromic DeviceElectrode

[0088]FIG. 3 Coloration Efficiency Comparison of Electrodes ResolvedAcross the Visible Spectrum.

[0089]FIG. 4 Spectroscopic Comparision of the Coloration Efficiency ofTungsten Oxide, Tungsten Nickel Oxide and Tantalum Nickel OxideElectrodes.

[0090]FIG. 5 Visible Spectrum Analysis of a Tantalum/nickel Oxide Film.

[0091]FIG. 6 Raman Spectra Comparison of Tantalum Nickel Oxide andNickel Oxide Films

[0092]FIG. 7 X-ray Difraction Spectra of a Tantalum-nickel Oxide Film.

[0093]FIG. 8 Visible Transmittance Spectrum Comparison of Bleached andColored States in an Electrochromic Device.

[0094]FIG. 9 Visible Transmittance Spectrum Comparison of ColoredTa-nio_(x) and Wo_(x) Films.

BEST MODE FOR CARRYING OUT THE INVENTION

[0095] One of the problems associated with electrochromic materials tobe utilized in a practical electrochromic device is related to the speedwith which the device executes a cycle from minimum to maximum opticaldensity, or from maximum to minimum optical density, the so calledcoloring/bleaching cycle. The problem is particularly acute in materialsthat utilize the movement of cations larger than hydrogen to achievecharge balance within electrochromic materials cycling between bleachedand colored states. Since the performance of many of the anodicallycoloring electrochromic materials is rendered unsatisfactory by thepresence of water, a device incorporating such materials of necessitymust use counter ions larger than protons.

[0096] While the speed of a coloring/bleaching cycle can be altered oversome range by increasing the voltage across the electrodes of thedevice, increased voltage is associated with decreasing lifetimes forthe device. Even under the condition that this decreased lifetime isacceptable, because of the development of a space charge region in thedevice during coloring, gains in coloring/bleaching cycle rates can onlybe modest when attempting to drive coloring of an electrochromic layerto an optical density exceeding 25% absorption by increasing theoverpotentials of the driving voltage source.

[0097] The material of the present invention is an anodically coloringelectrochromic material that addresses these problems. Using thedefinitions of complimentary electrochromic materials developed above,the material of the present invention exhibits electrochromic behaviorthat is complimentary to some commonly employed cathodically coloringelectrochromic materials, such as tungsten oxide-based electrochromicmaterials. Without wanting to be bound by theory, the material of thepresent invention is believed to undergo electrochromic coloring whenthe material undergoes oxidation with the accompanying migration ofcations out of it. The material of the present invention exhibits ionmobilities that exceed other prior art anodically coloring materials bya factor of 1-2 orders of magnitude, the rate seen for other materials.

[0098] Metal oxide films were prepared according to the process of thepresent invention having Ta incorporated into the oxide film to a levelof from about 5 to about 60 atomic % relative to the amount of nickelincorporated into the film. These concentrations were verified by X-rayPhotoelectron Spectroscopy (XPS), performed using a Physical ElectronicsPHI 5600 (a commercially available piece of analytical equipment) andstandard procedures well known to those skilled in the art of ESCAanalysis. The results of the XPS analysis show that Ta and Ni areincorporated into the oxide films of the present invention in the atomicratio indicated.

[0099] The material of the present invention is characterized by thepresence of tantalum substitutionally incorporated into an amorphoussolid solution of predominantly nickel oxide. While any amount oftantalum may be incorporated into a nickel oxide film, tantalumincorporated at an amount of between about 5 at. % and 95 at. % ispreferred. More preferred is tantalum incorporated to a level of betweenabout 5 at. % and 60 at. %. X-ray diffraction spectroscopy (XRD) andRaman Spectroscopy (Raman) performed on films of the material of thepresent invention are consistent with such a characterization.

[0100] XRD data obtained from samples of the tantalum doped nickel oxidematerial was gathered using a Scintag X-1 diffractometer. The data thusobtained was subjected to θ/2 q analysis, as will be familiar to thoseskilled in the art. The results of a typical XRD analysis obtained fromfilms of the present invention Ta/Ni oxide is shown in FIG. 7.

[0101] With reference to FIG. 7, the XRD analysis shows that thematerials of the present invention lack the long range order that isobserved in films of prior art metal oxide electrochromic materials. XRDanalysis of materials of the present invention is also not consistentwith a material having separate domains of the different crystallinemetal oxides as an admixture. The data illustrates that this is not amixture or phase separated mass of stoichiometric materials.

[0102] A Raman analysis, comparing films of a prior art crystallinenickel oxide and films of the present invention tantalum/nickel oxidealso shows that the two materials are distinctly different. Ramananalysis was performed using a quasi-backscattering geometry using as anexcitation source the 514.5 nm line from a 150 mW Ar ion laser sourcewhich was focused to a 5 mm×100 μm area. The signal was dispersed by aSpex 0.6M triple spectrometer and detected with aliquid-nitrogen-cooled, high-resolution, charge-coupled-device detectorarray. Both the spectral resolution and the accuracy in the Raman shiftare estimated to be ˜2 cm⁻¹.

[0103] Raman spectroscopic comparison of a crystalline nickel oxide filmand a Ni/Ta oxide film prepared according to the present invention arepresented in FIG. 6. This spectral comparison demonstrates clearly thatnickel exists in different environments in the two materials. It alsodemonstrates that the tantalum is incorporated into a single amorphousphase in the films of the present invention rather than as an admixtureof separate tantalum oxide and nickel oxide phases.

[0104] Taken together, the Raman and XRD analysis demonstrate that thetantalum/nickel oxide films of the present invention are amorphous, andnot a mixture of separate tantalum oxide and nickel oxide domains. Thisdata is also consistent with the view that Ta is substitutionallyincorporated into an amorphous nickel oxide.

[0105] The material of the present invention may be made by any means asis known in the art for applying a film or coating to a substrate. Thus,for example, the tantalum doped nickel oxide may be made by sputtering,reactive sputtering, vacuum evaporation, vapor deposition, chemicalvapor deposition, spray drying, precipitation (particularly sol-geltechniques) or application of a metal coating followed byelectrochemical oxidation of the coating. In all cases, once an oxidefilm has been formed on the electrode it is converted into anelectrochromically active material by injection of a counter ion speciesinto the film. In some methods of film preparation, such aselectrochemical deposition, successive voltametric sweeps can be used tosimultaneously form the film and inject counter ions into the newlyformed film.

[0106] The electrochromic properties of materials of the presentinvention were studied electrochemically using electrodes fabricatedwith a single electrochromic layer deposited on a conductive substrate.With reference to FIG. 2, electrodes 200 were constructed by applying alayer of the electrochromic material 202 onto a transparent conductingoxide (TCO) layer 203 which in turn resides on glass support 201. Thetransparent conductor was a sample of a transparent conductive oxideglass (TCO) obtained commercially. Transparent oxide conductors on glassare well known in the art, an example being indium doped tin oxide (ITO)glass. ITO glass was used in the example electrodes. One skilled in theart will appreciate that thin layers of metals, for example gold, mayalso be applied to a transparent support and serve as a transparent,conductive coating to glass.

[0107] The electrochromic material was deposited either by sputtering orby evaporative deposition, as detailed below.

[0108] Following deposition of an oxide used to form either ananodically or cathodically coloring electrochromic layer, the oxidelayer was injected with cationic species electrochemically. To inject acounter ion metal species into the oxide layer of the electrode thusformed, the conductive substrate of the electrode was placed in ohmiccontact with one pole of a source of voltage (emf source) which hasfacility for controlling the potential difference between two electrodesrelative to a reference electrochemical potential. Any voltage source asis familiar to one skilled in the art is suitable. Such a voltage sourceis, for example, the Arbin Battery Testing System.

[0109] Once connected to the controllable emf source, the electrode isplaced into a solution of a suitable salt containing the cation to beinjected, for example lithium perchlorate.

[0110] Suitable solvents for cation species salts are, for example,propylene carbonate, dried over sodium metal/sodium benzophenone orlithum/sodium amalgam and distilled under inert atmospheric conditions,such as will be familiar to one skilled in the art. Other suitablesolvents similarly dried will be readily apparent to one skilled in theelectrochemical arts.

[0111] With one pole of the emf source connected to the electrochromicelectrode, the other pole of the controllable voltage source isconnected to an electrode comprising a source of the cation species tobe injected, for example, lithium metal. The controllable voltage sourceis then cycled to a potential suitable to cause the counter ion speciesof interest to migrate into the oxide material, thereby producing theelectrochromic material. The controllable voltage source is cycledbetween voltages greater than those sufficient to cause migration intothe oxide material, and that sufficient to cause the material to achieveits maximum optically dense state. For example, when lithium is thecounter ion (species ejected from the oxide material during coloring),the voltage is cycled between about 1.0 Volt positive of a lithium metalreference to about 4.0 Volt positive of a lithium metal reference.

[0112] Voltage cycling is continued until a stable cyclic voltamagram isobtained. Those skilled in the art will comprehend that a stable cyclicvoltamagram indicates that the same quantity of charged species is beinginjected into the material during a reduction sweep as is being ejectedfrom the material during a oxidation sweep of the voltage cycle.

[0113] When the material exhibits a stable voltamagram, typically about10 cycles, the material is electrochemically cycled to either a bleachedor a colored state, removed from the electrochemical cell, and studiedspectroscopically as described above. These electrodes could also bereturned to the electrochemical cell for repeated cycling between thestates and removed again for further spectroscopic study.

[0114] The controllable emf source used affords the ability to measurethe current passed into or out of the electrochromic device facilitatingcalculation of coloration efficiency as well.

[0115] Electrodes were prepared containing single layers ofelectrochromic materials according to the present invention and othermaterials having electrochromic properties as comparative examples. Oncethe oxide layers had been deposited they were connected to an ArbinBattery testing system, and placed into a propylene carbonate solutionhaving lithium perchlorate present at between about 0.01 M and 10.0 Mconcentration. An Arbin Battery Testing system was used according to theprocedure described above to cycle the electrochromic materials betweenvoltages more positive than about 1.4 Volt positive of a lithium metalreference to about 4.0 Volt positive of a lithium metal reference untila stable cyclic voltamagram was observed. The material was then cycledto a voltage positive of lithium at which maximum current flow wasobserved during a reducing cycle (the reducing maxima) and held until nocurrent flow was observed, injecting the electrochromic material withlithium ions and driving it to a bleached state. The electrochromicmaterial thus prepared was then removed from the apparatus and used inspectroscopic testing. The electrochromic layer on the electrode wasalso subjected to a coloring cycle and the colored material was examinedspectroscopically as well.

[0116] Electrochromic devices containing electrochromic layers of thepresent invention were typically prepared by depositing successivelayers of material onto each other until all of the required layers werepresent in the device, as is well known in the art. To prepare a device,a combination of sputter deposition and vacuum evaporation was employed.

[0117] Thus, an electrochromic device having both an anodically coloringelectrochromic layer and a cathodically coloring electrochromic layerwas prepared by reactive ion sputter coating a layer of Ta doped NiOonto a sample of a transparent conductive oxide coated glass (TCOglass). By way of example, films were prepared by placing a 4″ circularnickel metal target available from Superconductive Components, Inc. intoa Varian 3-gun sputtering chamber. Coupons of 99.99% Ta metal, fromESPI, having a surface area of 3 in² each, were placed on the exposedface of the nickel disk until between about 5% to about 75% of thesurface of the nickel disk was covered. The sputter target was placedabout 7 cm from the sputtering gun. A sample of an indium tin oxidecoated glass substrate having a resistance of 10 ohm cm, a material ofcommerce, was secured in the vacuum chamber a distance of 7 cm from thesputtering target. No active temperature control of the substrate wasemployed. The chamber was sealed and evacuated to about 0.001 millitorr.Once the chamber pressure had been stabilized, water free oxygen wasadmitted to the chamber to establish a pressure of about 10 millitorr.The substrate temperature was measured during the deposition using athermocouple. The temperature of the substrate during deposition wasfound to be typically 40-50° C. Deposition of nickel/tantalum oxide wascarried out by supplying Rf power to the sputter guns at a power densityof about 150 watts across the sputter target. Power was maintained atthis level until a film of typically 150 nm was formed on the substrate.The film thickness was monitored throughout the deposition using anInficon thickness monitor. The ratio of Ta:Ni:O contained in thedeposited film was measured by XPS according to the procedure describedabove.

[0118] Following this, a source of pure lithium metal was heated undervacuum and a film of lithium metal was deposited onto the exposed faceof the TaNiO electrode. As the lithium formed on the surface it wastaken up by the material giving the lithium metal injectedelectrochromic material of the present invention.

[0119] After sufficient lithium metal had been injected into the counterelectrode thus formed, a layer of LiAlF4 was vacuum evaporated onto theexposed face of the counter electrode, forming a layer of electrolyte. Alayer of tungsten oxide was then evaporated onto the exposed face of theelectrolyte layer and a TCO layer was sputter coated onto the exposedface of the upper electrochromic layer. The TCO layer was depositedusing a target of pure indium tin oxide and sputtering techniques as arefamiliar to one of ordinary skill in the art.

[0120] It will be apparent to one skilled in the art that as analternative to Rf sputtering, DC sputtering techniques could be equallywell employed to produce the subject electrochromic materials and TCOlayer.

[0121] Following counter ion injection, the films were handled only in acontrolled environment to preclude their exposure to water and oxygen.While examples are disclosed below in which Li⁺ ions are used as counterions, other ions, for example Na⁺, H⁺, and K⁺, and others well known inthe art may also be employed and still be within the scope of thepresent invention material.

[0122] Optical spectroscopy measuring the electrochromic properties ofthe films and electrochromic devices of the present invention andcomparative examples was carried out using an Ocean Optics 001 Base32spectrophotometer.

[0123] The Spectral response and coloration efficiency compatiblebehavior of electrochromic materials was determined by comparing thetransmission spectra of a sample of a film or a device with a “blanksample.” A “blank sample” transmission spectra was obtained as abaseline by recording the light intensity as the spectrometer wasscanned through the range of wavelengths of interest with no samplepresent in the optical path of the spectrometer. In general, opticalspectra were obtained over the wavelength of about 300 nm to about 1400nm. The intensity of the light impinging on the detector was recorded ateach wavelength across the spectrum as is well known in the art ofobtaining a transmittance spectra. This procedure was repeated for afilm or device, and the transmittance values for the subject film ordevice at a given wavelength were offset from a 100% line at thatwavelength as established by the “blank” spectrum. In all cases thetransmission spectrum was corrected for the response of thephotodetector of the apparatus across the spectral region scanned.

[0124] The present invention may be further understood by reference tothe following examples, which are provided for the purpose ofillustration and not limitation of the scope of the present invention.

[0125] In the first set of examples, electrodes having a single layer ofelectrochromic material were prepared according to the above describedprocedures. Electrodes having anodically coloring electrochromic layerswere subjected to electrochemical coloring and bleaching using theprocedure described above. All example electrodes were examined in thebleached and colored states. The electrodes were cycled between bleachedand colored states using the procedures described above. All analyticalwork was carried out using the above described procedures.

[0126] Example Electrodes Containing a Single Layer of ElectrochromicMaterial

EXAMPLE 1

[0127] A 5 atomic % tantalum doped nickel oxide film was deposited ontoITO glass from a 4″ nickel sputtering target having 6% of its surfacecovered with tantalum metal coupons. Spectroscopic analysis of theresulting film showed that tantalum was substitionally incorporated intoan amorphous nickel oxide matrix. The film was subjected to lithium ioninjection according to the electrochemical procedure described above. Itwas found to have reversibly incorporated 5 mC/cm² lithium ions. Thelithiated film was subjected to coloration/bleaching cycles in anelectrochemical cell. The colored film was examined spectroscopicallyaccording to the above detailed procedure and found to have a peakwavelength coloration efficiency of 20 cm²/C at 400 nm.

EXAMPLE 2

[0128] An amorphous film comprising 25 atomic % tantalum doped intonickel oxide was deposited onto ITO glass from a 4″ nickel sputteringtarget having 30% of its surface covered with tantalum metal coupons.The deposition was carried out according to the procedure describedabove. The composition and nature of the film was confirmed byspectroscopic analysis as described above. The film was subjected tolithium ion injection according to the electrochemical proceduredescribed above. It was found to have reversibly incorporated 15 mC/cm²lithium ions. The lithiated film was subjected to coloration/bleachingcycles in an electrochemical cell. The colored film was examinedspectroscopically according to the above detailed procedure and found tohave a peak wavelength coloration efficiency of 55 cm²/C at 400 nm.

Example 3

[0129] An amorphous film containing 60 atomic % tantalum doped intonickel oxide was deposited onto ITO glass from a 4″ nickel sputteringtarget having 72% of its surface covered with tantalum metal coupons.Deposition was carried out according to the procedure detailed above.The composition and nature of the film was confirmed by spectroscopicanalysis as described above. The film was subjected to lithium ioninjection according to the electrochemical procedure described above. Itwas found to have reversibly incorporated 10 mC/cm² lithium ions. Thelithiated film was subjected to coloration/bleaching cycles in anelectrochemical cell. The colored film was examined spectroscopicallyaccording to the above detailed procedure and found to have a peakwavelength coloration efficiency of 25 cm²/C at 400 nm.

COMPARATIVE EXAMPLE 4

[0130] An ordered nickel oxide film having an average stoichiometerycorresponding to NiO_(1.5) was deposited onto a sample of indium tinoxide glass from a 4″ nickel sputtering target according to theprocedures described above. The composition and nature of the resultingfilm was confirmed by spectroscopic analysis as described above. Thefilm was subjected to lithium ion injection according to theelectrochemical procedure described above. It was found to havereversibly incorporated less than 5 mC/cm² lithium ions. The lithiatedfilm was subjected to coloration/bleaching cycles in an electrochemicalcell. The colored film was examined spectroscopically according to theabove detailed procedure and found to have a peak wavelength colorationefficiency of 10 cm²/C at 400 nm.

COMPARATIVE EXAMPLE 5

[0131] An ordered tungsten oxide film having a stoichiometerycorresponding to WO_(2.9) was deposited by reactive ion sputtering ontoa sample of indium tin oxide glass from a 4″ tungsten sputtering targetaccording to the procedures described above. The composition and natureof the resulting film was confirmed by spectroscopic analysis, asdescribed above. The film was subjected to lithium ion injectionaccording to the electrochemical procedure described above. It was foundto have reversibly incorporated less than 15 mC/cm² lithium ions. Thelithiated film was subjected to coloration/bleaching cycles in anelectrochemical cell. The colored film was examined spectroscopicallyaccording to the above detailed procedure and found to have a peakwavelength coloration efficiency of 55 cm²/C at 650 nm.

COMPARATIVE EXAMPLE 6

[0132] A film containing 25 atomic % tungsten doped into nickel oxidewas deposited onto ITO glass from a 4″ nickel sputtering target having30% of its surface covered with tungsten metal coupons. The compositionand nature of the film was confirmed by spectroscopic analysis, asdescribed above. The film was subjected to lithium ion injectionaccording to the electrochemical procedure described above. It was foundto have reversibly incorporated 20 mC/cm² lithium ions. The lithiatedfilm was subjected to coloration/bleaching cycles in an electrochemicalcell. The colored film was examined spectroscopically according to theabove detailed procedure and found to have a peak wavelength colorationefficiency of 30 cm²/C at 400 nm.

[0133] Films made according to Examples 2, 5, and 6 were subjected to acoloration cycle in an electrochemical cell according to the proceduredescribed above. These materials were compared in their colored statesby optical spectroscopy. The results from the three electrodes are shownsuperimposed in FIG. 4. Examination of FIG. 4 shows that the material ofthe present invention has higher coloration efficiency than the tungstennickel oxide across the spectrum. Of equal importance, FIG. 4 shows thatthe material of the present invention has nearly the same colorationefficiency in the blue region of the spectrum as a tungsten oxidematerial exhibits in the green through red region of the spectrum.Additionally, FIG. 4 shows that the material of the present invention(tantalum nickel oxide) has an absorption curve which falls offsymmetrically with the rise in absorption seen in the tungstenoxide-based material. This demonstrates that an electrochromic devicefabricated with a tantalum doped nickel oxide electrode of the presentinvention and a tungsten oxide electrode would provide a shutter havinga flat absorption curve across the visible spectrum. This demonstratesbroadband optical complimentary behavior. Both of the tungsten oxide(cathodically coloring material) and the tantalum nickel oxide(anodically coloring material) have similar coloration efficiency. Inthis manner, the coloration efficiency complimentary behavior of thesematerials is demonstrated. Since both materials use lithium as a counterion, ionic species complimentary behavior is also demonstrated.

[0134] Electrochromic Devices

[0135] Electrochromic devices were fabricated that contained both anodicand cathodic coloring electrochromic layers. The devices were preparedby sequentially depositing the various layers required in the device.With reference to FIG. 1, example devices 107 were fabricated utilizinga commercial sample of ITO glass as support 100 and transparentconducting layer 101. Onto layer 101 was deposited a 1500 angstrom thicklayer of an anodically coloring electrochromic material as a counterelectrode layer 102. The material was deposited by sputtering from anappropriate target using the same technique as described above forelectrochromic electrodes prepared for electrochemical study. Thecounter electrode layer was infused with a sufficient quantity oflithium metal to supply the requirements of the device being fabricatedby vacuum evaporation from a lithium source.

[0136] A 10,000 angstrom thick LiAlF₄ electrolyte layer 103 was thendeposited onto the lithium infused counter electrode by vacuumevaporation from a pure source of LiAlF₄.

[0137] A 5000 angstrom thick cathodically coloring electorchromic layer104 comprising tungsten oxide was deposited onto the exposed face of theelectrolyte layer from a pure tungsten oxide source utilizing vacuumevaporation techniques as described above. Finally, a 3000 angstromthick top conductive layer 105 comprising indium doped tin oxide wasdeposited by sputtering from a pure ITO source. A top protective layer106 was not used in these example devices.

EXAMPLE 7

[0138] An electrochemical device was prepared according to the sequencedescribed above. The device of Example 7 was fabricated with an anodiccoloring counter electrode comprising 25 atomic % tantalum incorporatedinto nickel oxide using a target prepared as described above in Example2. The cathodically coloring electrochromic layer in this devicecomprised WO_(2.9), prepared as described above in Example 5. Thisdevice was tested by connecting it to an Arbin Battery Testing Systemand measuring the charge required to cycle it between colored andbleached states and measuring its visible spectrum when in the bleachedand colored states.

EXAMPLE 8 Comparative Electrochromic Device

[0139] A second electrochromic device was prepared according to thesequence described above. The device of example 8 was fabricated with ananodic coloring counter electrode comprising NiO_(1.5) prepared asdescribed above in Example 4. The cathodically coloring electrochromiclayer in this device comprised WO_(2.9), prepared as described above inExample 5. This device was tested by connecting it an Arbin BatteryTesting System and measuring the charge required to cycle it betweencolored and bleached states and measuring its visible spectrum when inthe bleached and colored states.

[0140] Visible spectroscopy was used to compare the electrochromicdevices prepared according to Examples 7 and 8 in both the colored andbleached states. The results for the two devices in each state are showsuperimposed in FIG. 8.

[0141] With reference to FIG. 8 it can be seen that in the red end ofthe spectrum, wherein absorption is primarly due to tungsten oxideelectrochromism, the device incorporating the tantalum doped nickeloxide electrode performs equally regarding maximum optical densitycontrast, and coloration efficiency. It can as well be seen that in thegreen/violet/ultraviolet region of the spectrum, wherein absorption isprimarily due to either the tantalum-doped nickel oxide of the Example 7device, or the nickel oxide of the Example 8 device, the tantalum-dopednickel oxide material has superior performance regarding maximumdensity, contrast, and coloration efficiency.

[0142] Although the present invention has been described with referenceto preferred embodiments, numerous modifications and variations can bemade and still the result will come within the scope of the invention.No limitation with respect to the specific embodiments disclosed hereinis intended or should be inferred.

1. A metal oxide capable of undergoing reversible lithium metalinsertion, said metal oxide being characterized upon lithium ioninsertion by electrochromism, lithium ion mobility under an applied emfthat is substantially similar to that of electrochromic tungsten oxide,and a lack of long range crystal structure, said metal oxide comprising:a first transition metal present as a stable, amorphous metal oxidematrix; and a second transition metal doped into said stable amorphousmetal oxide matrix
 2. The metal oxide of claim 1 wherein, said firsttransition metal comprises essentially nickel and said second transitionmetal comprises essentially tantalum.
 3. The metal oxide of claim 1,further comprising inserted lithium atoms.
 4. The metal oxide of claim2, wherein the atomic ratio of tantalum:nickel is in the range of about5:95 to about 95:5.
 5. The metal oxide of claim 3, wherein said metaloxide electrochromism is characterized by anodic electrochromiccoloring.
 6. The metal oxide of claim 4, further comprising insertedlithium atoms.
 7. A thin metal oxide film capable of undergoingreversible lithium metal insertion residing on a transparent substrate,said thin metal oxide film being characterized by displaying, uponlithium metal insertion, reversible anodically coloring electrochromism,said thin metal oxide film comprising: a first transition metal presentas a stable amorphous metal oxide matrix; and a second transition metaldoped into said stable amorphous metal oxide matrix.
 8. The thin metaloxide film of claim 7, wherein said first transition metal comprisesessentially nickel and said second transition metal comprisesessentially tantalum.
 9. The thin metal oxide film of claim 7, furthercomprising inserted lithium atoms.
 10. The thin film of claim 8, whereinsaid first transition metal is present in the thin film in the rangefrom about 5-95 atomic percent relative to said second transition metal.11. The metal oxide of claim 8, further comprising inserted lithiumatoms.
 12. The metal oxide of claim 10, further comprising insertedlithium atoms.
 13. A metal oxide film capable of undergoing lithiummetal insertion, said metal oxide being formed on a substrate byreactive oxygen ion sputtering from a target containing both tantalumand nickel and having an atomic ratio of Tantalum:Nickel of betweenabout 5:95 and about 95:5, and lacking any long range ordered structureas determined by XRD.
 14. The metal oxide film of claim 13, wherein theratio of Ta:Ni surface area in the sputtering target is between about5:95 and about 95:5.
 15. The metal oxide film of claim 13, furthercomprising inserted lithium atoms.
 16. The metal oxide film of claim 14,wherein said substrate is a glass bearing a transparent conductive oxidecoating selected from tin oxide and indium doped tin oxide.
 17. Anelectrode containing a lithium metal injected metal oxide film layer,said metal oxide film layer being formed on a transparent glasssubstrate with a transparent layer of a conductive oxide interposedbetween said metal oxide film layer and said transparent glasssubstrate, wherein said metal oxide film is formed by reactive oxygenion sputtering from a target containing both tantalum and nickel, andwherein said metal oxide film is subjected to a lithium metal injectionprocess, said metal oxide film being characterized by the followingproperties: an atomic ratio of Tantalum:Nickel of between about 5:95 andabout 95:5; an absence of any long range crystal structure as determinedby XRD; electrochromic behavior such that said metal oxide filmundergoes electrochromic coloring when connected to a cathode of an emfsource having sufficient potential to drive said coloring andelectrochromic bleaching when connected to the anode of an emf sourcehaving sufficient potential to drive said bleaching; and anelectrochromic coloration efficiency of greater than 5 cm²/C measured atabout 400 nm.
 18. The electrode of claim 17, wherein the atomic ratio ofTa:Ni and the amount of lithium metal injected into said metal oxidefilm layer are selected to yield a cathodically coloring electrochromicelectrode having broad band optical complimentary behavior, ionicspecies complimentary behavior, and coloration efficiency complimentarybehavior with a tungsten oxide electrochromic electrode.
 19. Anelectrochromic device, the device comprising: a glass support; a firstconductive layer; a tantalum nickel oxide layer wherein said layer isformed by sputtering and injected with lithium; an electrolyte layer; atungsten oxide layer, wherein said layer is formed by vacuumevaporation; and a second conductive layer in ohmic contact with saidtungsten oxide layer.
 20. The electrochromic device of claim 19 whereinsaid first and second conductive layers are indium doped tin oxideglass, and wherein said nickel tantalum oxide layer has a Ni:Ta ratio ofbetween about 95:5 and about 5:95.
 21. The electrochemical device ofclaim 19, wherein said nickel tantalum oxide layer and said tungstenoxide layer combination exhibit a broadband optical complimentarybehavior, an ionic species complimentary behavior, and a colorationefficiency complimentary behavior.
 22. An electrode containing a lithiummetal injected metal oxide film layer, said metal oxide film layer beingformed on a transparent glass substrate with a transparent layer of aconductive tin oxide interposed between said metal oxide film layer andsaid transparent glass substrate, and wherein said metal oxide film isformed by reactive oxygen ion sputtering from a target containing bothtantalum and nickel, wherein said metal oxide film is subjected to alithium metal injection process, said metal oxide film beingcharacterized by the following properties: an atomic ratio ofTantalum:Nickel of between about 5:95 and about 95:5; an absence of anylong range crystal structure as determined by XRD; electrochromicbehavior such that said metal oxide film undergoes electrochromiccoloring when connected to a cathode of an emf source having sufficientpotential to drive said coloring and electrochromic bleaching whenconnected to the anode of an emf source having sufficient potential todrive said bleaching; and an electrochromic coloration efficiency ofgreater than 5 cm²/C measured at about 400 nm.
 23. The electrode ofclaim 22 further comprising an atomic tantalum:nickel ratio of betweenabout 5:95 and about 60:40.
 24. The electrode of claim 22 wherein theconductive tin oxide is indium tin oxide.